Neutron detection



Patented June 27, 1 9 50 2,512,789 NEUTRON nnrncnon Kenneth C. crumrlna'Bellalre, Ten, asslgnor to The '1 Company, New York, N. Y., a corporation of Delaware I Application October so, 1945, Serial No. 625,597

f A solid, smooth layer of a neutron reactive substance such as boron or one of its compounds could be used as a, lining for the detector cathode. Such a layer would present a smooth surface to the gas to be ionized and while detectors of this type have been used with some success, I have found that by providing a cathode coating of a substance such as boron carbide in which the alpha particle emitting surface is uneven or saw-toothed in cross section rather than smooth the surface area from which the alpha particles are emitted can be made several times as great as the area, of the cathode on which the layer is chambers and proportional counters, the last I named has proven to be very suitable for detecting neutrons. In my copending application Serial No. 511,516, filed November 24, 1943,-n0w U. S. Letters Patent No. 2,462,471, granted February 22, 1949, a proportional counter is disclosed which isvery sensitive to neutrons and which is insensitive to gamma radiation. The said application clearly describes the difierent characteristics of the three types of radiation detectors. 7 In the co'pending applicationof Gerhard Herzog and myself, Serial No. 511,990, filed November 27, 1943, now U. S. Letters Patent No. 2,443,- 731 granted June 22, 1948, other forms of proportional counters are disclosed in connection with neutron well logging.

A neutron detector or proportional counter usually comprises a housing adaptedto be sealed and containing a, cathode, an anode and a gas filling. Since neutrons do not ionize, it is necessary in order to detect them, to use an intermediate reaction by which the neutrons release ionizing particles which in turn act on the gas of the proportional counter. Such reactions are placed- Consequently, there is a much greater chance for alpha particles to be emitted than were the surface smooth.-

In carrying out the invention in one form, boron carbide in the form of powder in which the grain size may vary up to three mils is mixed with an adhesive, preferably a synthetic resin type cement such as- Duco cement which is thinned withacetone and its drying speed suitably retarded by a small and controllable amount of amyl acetate. In the following discussion, where minu particles are mentioned, particles or powder having a .grain size of up to 3 mils and averaging about 1 mil are intended. The boron carbide powder in the adhesive binder is then sprayed onto the inner surface of the metal cathode sheet which may be in the form of a cylinder. During the spraying operation the coating is intermittently observed by means of a microscope and after the desired average thickness, preferably about one mil, has been obtained and the adhesive dried, the cathode is ready to be placed in the detector housing. In another embodiment a layer of the adhesive may be placed known to occur with lithium and boron, for example.

A proportional counter can either be filled with gas (such as boron trifluoride) which the two procedures mentioned, it has been found that the coating of the cathode with a layer of the neutron reactive substance is more satisfactory than the use of a neutron reactive gas since, with the "coating" method the resulting detector has a higher efliciency for detecting slow neutrons than the gas-filled detector of comparable dimensions, operating voltage, etc.

on the cathode surface either by spraying, dipping or wiping it on and then the dry powder comprising the neutron reactive substance sprayed or dusted onto the adhesive. T

For a, better understanding of the invention reference may behad to the accompanying drawing in which:

Figure a vertical sectional elevation through a simple form of neutron detector or proportional counter,

Figure 2 is a greatly enlarged cross section showing the neutron reactive coating orlayer on a portion of the cathode, and

Figure 3 is a curve showing how the efficiency of the detector varies with the thickness of the layer.

In the detector as illustrated in Figure 1 a cathode l0 and an anode l2 are mounted within a housing It which may be a glass envelope or a some container of brass, aluminum or the like. 1: glass is used, one should be certain that it no compound of boron or lithium. The. cathode ll as shown am the form of a thin walled cylinder which may be formed of a sheet of brass or any other suitable metal and the anode II is in the form of a fine wire,'preferably of tungsten and, about three mils in diameter, stretched coaxially through the cathode and secured at its ends. As

showninl'igurelthe anodeismaintainedata positive potential with respect to the cathode and a fairly high resistance R-is connected in series with the electrodes and a source of high potential. It has been found that for a proportional counter of this type a potential difference of the order of 1000 volts is's'atisfactory" 'Ihe inner surface It oi the cathode cylinder II is coated with boron carbide preferably as described hereinbefore, e. g., by spraying boron carbide particles or powder in an adhesive binder onto thesurface until an average thickness of about one mil has been obtained. The housing or envelope I4 is filled with a gas and-whi1e various gases are more or less satisfactory, methane under a pressure of about two inches of mercury has been found suitable.-

A neutron in penetrating the cathode cylinder Ill and the neutron reactive coating I! may release an alpha particle vdue to the nuclear reac-,

tion and this particle may then cause ionization of the gas, thus producing a discharge or pulse in the detector. A voltage pulse across the resistance R due to the detector discharge is thus pro-' .the layer and could not enter the gas of the counter and produce ionization. On the other hand,

an increase in thickness of the boron coating over this amount is generally'not harmful since the absorption of neutrons in the coating is very small. However, in case a multiple counter is used, 1. e., one in which a plurality of cathodes and anodes are disposed within a single housing such as is described, for instance, in the aforementioned application, Serial No. 511,990, a neu-' tron may have to pass through several cathodes with their coatings and the absorption of the neutrons may'then become appreciable.

In the discussion in the foregoing paragraph it was assumed that the boron carbide coating was in the form of a solid uniform layer. It has been found that where boron carbide powderor particles are sprayed onto a smooth surface, the resulting surface of the boron carbide in contact with the gas filling is not smooth but when viewed in the microscope is quite rough and saw-toothed during the spraying,

r. v thereisnoameciableloas duetoabsorptioninthecoating. Thisnaturally increases the eflloiency of the'device forthe detection ofneutrons.

lnl'igurezanenlargedcrosssectionofaportionofthecathode llisshowmthecoatingof the boron carbid I. being disposed on the sur-' -faceofthecathodosothatthemfaceotthe coatingincontectwiththegasflllingwillberelativelylarge.

l'igureaisacurveshowingtheeflldencyof neutrondc'tectorsinwhichthecathodeisprovidedwithacoatingorlayerotboroncarbide. Thelowercurveischaracteristicforacounter whichhasasingletubeorplateasthecathode layer or layers of boron carbide. For a single tube counter this absorption is not too important 'because the part of the neutron beam which is notused by the onecathode is lost-as far as the measurement is concerned. With a multiple tube counter it is important that the neutron beam notbeweakenedmorethannecessaryintheflrst cathode so that the following cathodes will re ceive as high a neutron intensity is. mime, as has been brought out in a previous paragraph. It will be noted that the emciency of either the single or multi-tube counter does not continue to increase with increasing thickness of the boron but on the contrary, reaches a peak and then in cross section. Due tothis unevenness it has falls ofl. It has been found that a boron carbide coating which averages one miiin thickness is quite satisfactory.

The following method has been found suitable for applying the coating to the detector cathode. The tube or cathode cylinder is mounted in the chuck of a lathe and around the end of the cylinder and coaxial therewith is wrapped a small, thin strip of t plastic which projects beyond the endof the cathode cylinder. A spray gun is mountedso as to enter the rotating cylinder and plastic test strip and to spray a jet of the boron carbide particles and adhesive onto their inner surface.v After the spray gun has made a few in and out 'of the rotating cylinder, the machine is stopped, the test strip removed and a cross section of the coating observed under a microscope in comparison with a standard. In'this manner the desired thickness of the coating can be quite easily obtained.

Instead of mixing the boron carbide powder with the adhesive and then spraying the mixture onto the cathode surface, the cathode may be coated in any suitable manner with the adhesive and then while the adhesive is wet or tacky, the coating sprayed or dusted with the dry powder. This method has the advantage that the individual boron carbide particles are not covered with a layer of the binder or adhesive which layer, of course, absorbs some of the alpha particles which are emitted under the action of the neutrons.

Although boron carbide has been referred to as the preferred cathode-coating material, other substances such as for example boron, lithium and the like can be used if desired.

While the cathode itself has been described I as formed of a solid sheet of a suitable metal, it is to be understood that it may be formed of perforated or expanded metal, metal gauze or the like and it may also, of course, take other forms than that of a fiat plate or a cylinder.

Although the detector or counter has been described as having a cylindrical cathode, it is to be understood that the invention also contemplates the use of cathodes of other forms such as a plurality of slightly separated parallel discs or plates containing holes through which one or more anode wires are stretched. A gamma radiation detector constructed in this manner is disclosed in the U. S. Letters Patent of D. G. C. Hare, No. 2,397,071, granted March 19, 1946. In another form the anode wires may be disposed between and parallel to the separated, fiat cathode plates. A gamma ray detector of this type is shown in the U. S. Letters Patent of D. G. C. Y

Hare and Gerhard Herzog, No. 2,397,074, granted March 19, 1946.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof and, therefore. only such limitations should be imposed as are indicated in the appended claims.

Iclaim:

1. In a neutron detector, a housing, a wire anode. a cathode and a gas in said housing, said cathode comprising a sheet-like metal member havin a coating on at least part of its surface of a substance capable, when bombarded by neutrons, of ejecting alpha particles to ionize said gas. saidcoating being composed of minute particles of said substance in an adhesive binder and having a surface area several times the area of the coated portion of said member, said particles averaging 1 mil in grain size.

2. In a neutron detector, a housing. a wire anode, a cathode and a gas in said housing, said cathode comprising a sheet-like metal member having a coating on at least part of its surface of particles of boron carbide in an adhesive binder, said particles averaging one mil in grain I size.

3. In a neutron detector, a housing, a wire anode, a cathode and a gas in said housing, said cathode comprising a sheet-like metal member having a coating on at least part of its surface of particles of boron carbide in an adhesive binder, said particles averaging one mil in grain size, and said coating having an average thickness of from one-tenth mil to three mils.

4. In a neutron detector a housing, a wire anode, a cathode and a gas in said housing, said cathode comprising a sheet-like metal member having a coating on atleast part of its surface of particles of boron carbide, said coating having an exposed surface which in cross section will present myriads of fine peaks and valleys.

5. In a neutron detector a housing, a wire anode, a cathode and .a gas in said housing, said cathode comprising a sheet-like metal member having a coating on at least part of its surface of a substance capable, when bombarded by neutrons, of ejecting alpha particles'to ionize said gas, the exposed surface of said coating appearing substantially uniform and smooth to the naked eye but in cross section under the microscope appearing rough, having myriads of minute peaks and valleys.

KENNETH C. CRUMRINE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,220,509 Brons L. Nov. 5, 1940 2,233,917 DeBoer et al. Mar. 4, 1941 2,250,189 Bachrnan July 22, 1941 2,288,718 Kallmann et a1 July 7, 1942 2,368,060 Wooten Jan. 23, 1945 2,440,167 Broxon et al Apr. 20, 1948 

